1. Field of the Invention
The invention relates to the field of welding of polyolefin-based articles. More particularly, it relates to a method of welding certain polyolefin-based articles by dielectric heating, wherein the articles exhibit properties improvements.
2. Background of the Art
Dielectric heating provides a method of contactless, direct, volumetric heating of dielectric materials, which are defined as materials that are capable of absorbing electromagnetic energy and which are also very poor electrical conductors. This is achieved by the application of an alternating electric field generated at the given frequency range to the dielectric material. The dielectric loss factor, also known as the dielectric loss index, is a measure of the energy loss in a material and is indicative of how well that material can be heated in the high frequency, alternating electric field. If a material has a relatively high dielectric loss index, it may be well-suited for subjection to dielectric heating. In such a case dielectric heating offers a clean and controllable process that eliminates problems associated with the application of direct heat to a variety of materials. Thus, the process has been found to be economically attractive for use in certain commercial and industrial manufacturing processes. Dielectric heating is typically carried out using radio frequency (RF) waves (3 kHz to 300 GHz), which include microwaves (300 MHz to 300 GHz). Together these wave ranges are referred to as high frequency (HF) waves.
One application of dielectric heating is referred to as radio frequency (RF) welding or sealing, also referred to as high frequency (HF) welding or sealing. In this application at least one surface of a sheet or film of a suitably lossy material (i.e., having a relatively high dielectric loss index) is welded or sealed, either to another surface of the same sheet or face of another sheet or film, in order to fabricate an article of some type. Suitably lossy materials may include those containing functional groups having dipole moments that are responsive to the high frequency electromagnetic waves. Examples of this may include certain specific polymers, such as polyvinyl chloride. Unfortunately, however, chlorinated compounds may present environmental or toxicological challenges that manufacturers would like to avoid.
Researchers have tried to find means to make a normally non-lossy material suitable for dielectric heating by incorporating some proportion of a second, RF responsive material therein. In International Publication No. WO/2002/088229 the dielectric heating of thermoplastic compositions included incorporating a molecular sieve with at least one interpolymer described as comprising (i) polymer units derived from at least one aliphatic olefin monomer having from 2 to 20 carbon atoms; and (ii) polymer units derived from (a) at least one vinyl or vinylidene aromatic monomer, or (b) from at least one sterically hindered aliphatic or cycloaliphatic vinyl or vinylidene monomer, or (c) from a combination of at least one vinyl or vinylidene aromatic monomer and at least one sterically hindered aliphatic or cycloaliphatic vinyl or vinylidene monomer, and optionally, (d) polymer units derived from at least one ethyleneically unsaturated polymerizable monomer other than that derived from (a), (b) or (c). An example thereof would be an ethylene-styrene interpolymer.
Another disclosure addressing use of zeolites is Japanese Patent Application No. 10-219048, which describes HF weldable polypropylene compositions consisting of fine metal oxide particles, ion-substituted zeolite, and polypropylene. The ion-substituted zeolite, in which a portion or all of the exchangeable ions are replaced with ammonium ions and silver ions, is used as a nucleating agent and chlorine scavenger. European Patent Application No. 193 902 proposes to use zinc oxide, bentonite clay, or crystalline and amorphous alkali or alkaline earth metal aluminosilicates as HF sensitizers for high molecular weight, high density polyethylene or ultrahigh molecular weight polyethylene. It is taught therein that the radio frequency sensitizers have to be essentially dried prior to use. European Patent No. 149 782 discloses compositions comprising silane-grafted polyolefins and a crystalline zeolite. The compositions are shapeable into articles which can be cross-linked after shaping by exposure to humidity and/or microwaves.
In fact, some of the most widely used thermoplastic polymers, such as polyethylene, polystyrene and polypropylene, are known to be insufficiently lossy to permit efficient dielectric heating. Other researchers have tried to make HF weldable compositions by combining such non-lossy olefins with polar acrylate esters or vinyl acetates. See UK Patent Application No. 2 177 974. Still another involves using blends of propylene-ethylene copolymers and an ethylene-alkyl acrylate copolymer. See International Patent Application WO 94/12569. International Application No. WO 00/69629 discloses HF weldable films made from a blend of a non-polar olefin polymer and a polar copolymer with carbon monoxide (CO). U.S. Pat. No. 3,336,173 discloses HF weldable polyethylene and polypropylene compositions obtained by the incorporation of long chain synthetic polyamide resin. U.S. Pat. No. 6,136,923 discloses thermoplastic compositions of α-olefin monomers with one or more vinylidene aromatic monomers and/or one or more hindered aliphatic or cycloaliphatic vinylidene monomers blended with polyvinyl chloride.
In view of the above, there is still a need for polymer compositions which are suitable as substitute materials for chlorine containing polymers, particularly polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC), which can be dielectrically heated. In particular, there is a need for such polymer compositions which can be formed into HF weldable structures, including, for example, a film a sheet, a foam, a profile, fibers, a molding, or a fabricated article.
Furthermore, there is also a need for a material that has desirable appearance and also acceptable or desirable mechanical properties. Such properties are frequently defined as requiring a weld failure that is cohesive in nature, and a weld strength for a part having a 10 mil (0.254 millimeter) thickness that is greater than 5 pounds per inch (lb/in, 0.88 Newtons per millimeter, N/mm), preferably greater than 7 lb/in (1.23 N/mm), preferably under welding conditions including less than or equal to 6 seconds welding time and certain optimized radio frequency welding conditions including power ranging from 80 to 100 percent (%).